The above described use of an electric-arc low-shaft furnace for the production of silicon and ferrosilicon is known. The principles are described, for example, in U.S. Pat. No. 4,820,341 and in earlier patents including 4,366,137, 4,364,974 and 4,389,493. The production of raw material bodies especially suitable for producing silicon or silicon alloys is described in U.S. Pat. No. 5,078,927, for example, and reference may also be had to U.S. Pat. No. 4,975,226. The production of briquettes is likewise described in U.S. Pat. No. 5,073,107.
Using these systems and the briquettes as described, the reaction in the first process stage can be described by the following equation: EQU SiO.sub.2 +3C=SiC+2CO,
This reaction in the first process stage is effected at a temperature of 1520.degree. C. and higher. The reaction in the second stage corresponds to the following equations: EQU Sio.sub.2 +2SiC=3Si+2CO, EQU Sio.sub.2 +C=SiO+CO,
and is effected at a temperature of 1800.degree. to 2000.degree. C. (see German Patent Document DE 20 55 564 and German Patent Document DE 30 32 720).
The compacts, i.e. the raw material bodies or briquettes must have chemical compositions corresponding to these equations and in general include a stoichiometric excess of carbon with respect to the reactions in which these compacts or briquettes play a role. In addition, the raw material compacts must have physical properties such that they satisfy the requirements of integrity along the path and the charge and remain intact from manufacture until they are introduced into the electric-arc low-shaft furnace, and in travel through the furnace until, of course, they participate in reaction with the molten quartz.
To produce such raw material compacts which satisfy the chemical and physical requirements, a number of processes have been proposed, including those identified above and those described in German Patent Documents DE 30 09 808, DE 34 25 716 and DE 39 39 018. For convenience, it is noted that DE 39 23 446 corresponds to U.S. Pat. No. 5,078,927, that DE 35 18 151 and 35 41 125 corresponds to U.S. Pat. No. 4,820,341, that DE 30 32 720 corresponds to U.S. Pat. No. 4,366,137, that DE 30 09 808 corresponds to U.S. Pat. No. 4,389,493, that DE 37 24 541 corresponds to U.S. Pat. No. 5,073,107 and U.S. Pat. No. 4,975,226 and that DE 30 23 297 corresponds to U.S. Pat. No. 4,364,974.
It is especially to be recognized as known that the porous and coke-like structure which is formed from the carbon and silicon carbide in the first stage within the briquette, contributes significantly to the silicon conversion because the silicon oxide formed in the second process stage and liberated as a gas would tend to be lost from the charge except that it taken up by the spongy coke-like structure of the briquettes and initially condensed so that it is entrained further in the charge with the briquettes and can be reacted within the pores of the compacted bodies to produce silicon.
Notwithstanding the fact that the briquettes as described contribute significantly to reduction in the loss of silicon monoxide and thus to greater silicon conversions, it has been found that further improvement is desirable.